The present invention relates generally to a test set for transient protection devices designed to test these devices in their operational environment. The test set of the present invention tests not only surge arresting circuits but also tests for wiring continuity in the circuit to be tested.
There exists numerous radio and telephonic communications facilities having transient protection devices. To ensure that the transient protection devices are functional without affecting the circuits they are supposed to protect, there exists a need for a compact, inexpensive, rapid and simple go/no go tester. Many of the transient protection devices, even for relatively small communications facilities have as many as 48 individual transient protection circuits that would need to be individually tested. This constitutes a single common entry panel which terminates wires from distant communications facilities. A signal entry panel can accommodate up to 24 four wire channels and there can be more than one signal entry panel per each of the communications facility. The transient protection devices when installed are inserted between binding posts and the circuits to be protected.
Each of the individual transient protection circuits is comprised of a low pressure, gas filled spark gap on the input side and a bipolar zener device on the output side with low value of inductance (100 microhenry) in between. For example, the bipolar Zener device may be such devices as manufactured by General Semiconductor Corp., Tempe, AZ, under the tradename "Transzorb." Transient protection is achieved from a single wire to ground, usually the frame of the signal entry panel. The spark gap operates by providing a momentary short circuit to ground whenever its breakdown voltage is exceeded. For a transient with a slow rise time, the breakdown of the spark gap is typically 850 volts. For transients with a fast rise time, the breakdown voltage is considerably higher. The bipolar zener devices operate by clipping the positive and negative excursions of the offending transients. In this case, the clipping voltage is 140 volts .+-.20% for both polarities. Thus, for transient protection circuits with spark gaps and bipolar zener devices it is necessary to conduct two tests per circuit.
One problem that exists when testing amplitude limiting devices, such as bipolar zener devices, is that it is necessary to subject them to voltages in excess of their rated breakdown voltages. Typically this is 150 volts. A DC source with sufficient voltage could be provided to apply voltage to the transorb and measure the voltage at which amplitude limiting occurred. There are two major disadvantages to this: (1) the amplitude limiting device is a bipolar device so the operator conducting the test would have to reverse the polarity, thus increasing the time required to conduct the testings; and (2) there would be a steady high voltage DC potential on the test lead which is a potential safety hazard. Another method would be to use a 60 HZ power line AC signal to obtain high voltage breakdown for both polarities. Again, this would constitute a safety hazard. The present invention circumvents the safety hazard by providing an alternating positive and negative polarity bipolar pulse train. The test leads can be held, one in each hand and not feel an electric shock. Another advantage of the present invention is that less power is consumed even under load conditions and therefore reduces heating within the instrument case. Another advantage of the present invention is that the utilization of a bipolar pulse train in combination with the technique for testing spark gap breakdown is also used to determine if spark gaps within a transient protection device assembly are good. The bipolar pulse train is processed to determine whether there are any short circuits to ground. Without the bipolar pulse train, short circuits to ground would deceive the spark gap breakdown detector into indicating that a spark gap had broken down correctly.
The present invention provides a new and novel technique for testing spark gaps. As indicated above, during the testing of the bipolar zener devices, one half of the spark gap test will have been performed and short circuits to ground will have been ruled out. The conventional technique used for in-circuit testing of spark gaps requires a high voltage pulse generator, a fast sweep oscilloscope and a polaroid camera. A high voltage pulse, usually single shot, is applied between input and ground. The voltage-time waveform at the output is recorded on polaroid film. The disadvantage to this technique, besides the necessity for the above listed equipment, is that a very experienced technician is needed to perform the test and to interpret the visual display. Another major disadvantage is that it is necessary to have physical access to the circuit output. The present invention circumvents these disadvantages in that the operator need not be experienced nor is it necessary to interpret visual displays (other than recognizing a color of an indicator LED). Additionally, the circuit need not be removed nor is it necessary to have access to the output of the circuit under test.
It is therefore one object of this invention to provide a method for testing transient protection circuits that is simple, rapid and safe.
It is another object of this invention to provide a method for testing transient protection circuits that does not require physical access to the circuits under test.
It is a further object of this invention to provide a device for testing transient protection circuits that is compact, inexpensive and safe.
It is still another object of this invention to provide a device for testing transient protection circuits that does not require high voltage-high power power supplies and does not require physical access to the circuits under test.
It is still a further object of this invention to provide a device for testing transient protection circuits that does not require an experienced operator.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.